ABSTRACT This implementation science K01 research and training grant aims to optimize the delivery of pre-exposure prophylaxis (PrEP) for pregnant and postpartum women. Pregnancy is a high-risk time for HIV acquisition. As prevention of mother-to-child transmission (PMTCT) programs become more effective, a greater proportion of new infant infections will be due to incident infection during pregnancy. PrEP is an effective, female-controlled, evidence-based intervention that is recommended during pregnancy in high-risk regions in both World Health Organization and Kenyan guidelines. An implementation science approach will be needed to improve operationalization and scale-up of PrEP delivery during pregnancy. An ongoing 20-site R01 trial (MPI: John- Stewart, Baeten) and a recently completed 16-site project (MPI: John-Stewart, Baeten) are the first and largest projects to deliver PrEP during pregnancy within antenatal care clinics (ANC). The proposed K01 will leverage the experiences of health care workers (HCW) and the heterogeneity between sites, to identify and test ways of improving PrEP delivery. This project?s research aims (R1-3) are to R1) identify determinants of successful PrEP delivery during ANC using mixed-methods, R2) select and test specific implementation strategies to improve PrEP delivery fidelity and penetration using interrupted time series analyses, and R3) quantify the budget impact of modifications to PrEP delivery during ANC and understand policymaker decisions. The research aims of this study will allow Dr. Wagner the opportunity to meet her training aims (T1-6), including: T1) learning implementation science frameworks, outcome measurement, and implementation strategy identification and testing, T2) learn quasi-experimental research designs, T3) conduct budget impact analyses, T4) understand policymaker decision-making processes, T5) learn about responsible conduct of research with pregnant women, and T6) build her scientific record and transition to independence. These research and training goals will prepare Dr. Wagner to transition to independence; she will plan to submit an R01 cluster randomized trial to test a package of implementation strategies to improve successful delivery of PrEP during ANC. Dr. Wagner will complete her K01 career development training at the University of Washington, which is a superb environment for training in implementation science and health economics, with a strong >25 year history of collaboration with Kenyan institutions, and with outstanding mentorship from implementation scientists, PrEP experts, leaders in HIV prevention during pregnancy, and policymakers.